You are right that whether to use "so many" or "so much" depends upon whether the thing in question is countable. It's the difference between "I can't believe she has so many cats!" and "I can't believe there's so much work to do!" The negative form should work the same way, and I think it does most of the time. For example, "There aren't (are not) so many cars on the road at this hour" (because cars are countable), and "There's not so much controversy about the dress code anymore" (because the amount of controversy is not countable). I (a US speaker of English) would not say "it's not so much" about someone with 5 cats. Cats are countable, so "much" seems strange. I could say "He's got 5 cats. That's not so many." There's another usage for "not so much" that doesn't seem to have a positive form, and that's when you give a reason for doing something. You can say "It's not so much that it's on sale; I'm buying the book because it's by my favorite author." Here, I mean that while a lower price may have had some effect on my decision, the main reason I decided to buy the book is because my favorite author wrote it. To my knowledge, there's no way to say this using the positive "so much" without the "not." By the way, all these forms are fairly informal. Strictly speaking, we shouldn't use "so" in this way without a subsequent "than" or "that." These are more formal examples: - There is so much more work than we can complete. - There is so much work that the workers are beginning to complain. - There's not so much work that we need to hire more people. - There are so many more cars here than there are in the countryside. - There are so many cars that no one can find a place to park. - There are not so many cars that we will have to wait for the next ferry.